UWM subscribes to a lot of resources that have what would be considered primary source visual information - newspapers like the NY Times going back to 1851 and ArtStor. This is a selective list of them.
ArtStor - a digital library of over 1 million images in the areas of art, architecture, the humanities, and social sciences with a set of tools to view, present, and manage images for research and pedagogical purposes.
American Mosaic - a full text database of reference sources and primary documents (including video and photographs) focusing on the history and culture of African Americans, Latino Americans and American Indians.
Early English Books Online - over 125,000 titles listed in Pollard & Redgrave's Short-Title Catalogue (1475-1640), Wing's Short-Title Catalogue (1641-1700), and the Thomason Tracts (1640-1661). by William Caxton, through the age of Spenser and Shakespeare and the tumult of the English Civil War.
Example: if you need to make an insignia for a costume for a knight in 1617, this page of examples might come in handy
Eighteenth Century Collections Online - a comprehensive digital edition of The Eighteenth Century, the world’s largest library of the printed book on microfilm. Access includes the original base collection of 135,000 volumes (ECCO I) plus a 45,000 volume supplement (ECCO II).
Example: They have an image gallery
The Gilded Age - provides insight into the key issues that shaped America in the late nineteenth century, including race and ethnicity, immigration, labor, women's rights, American Indians, political corruption, and monetary policy. The Gilded Age brings together primary documents and scholarly commentary into a searchable collection that is the definitive electronic resource for students and scholars researching this important period in American history.
Example: You can limit the search to illustration/photograh
Searching Google for images is quick and easy. The problem is that that the descriptions for them are often unreliable. Libraries and archives the world over had been digitizing images and providing accurate information about their provenance for about 10 years. This is a selective list of such resources that can be used to pin down what clothes might have looked like across time. These sites also provide information on re-using the images legally. For example, if you decide to reproduce and image that you used as a guide to the costumes in a program, you need permission. Usually it's ok, but you need to ask first.
American Memory Project - the Library of Congress' huge digital collection of images, film, audio, scores, etc. documenting life in the US.
Cities Around The World - a collection of photographs taken by a former professor at UWM. Most are of buildings and structures, but many include people and because they are accurately dated, you can get a sense of what clothes were worn in that city at the time.
Europeana - a portal that searches the digital collections of libraries, archives, and museums all around Europe.
New York Public Library -
The Albert Kahn Museum and Gardens in Paris.
Scottish Registry of Tartans - what it says on the tin!!
A Tonic to the Imagination - costume designs for stage and screen by the B. J. Simmons & Co. 1889-1959
UW Digital Collections - primarily images and documents that tell the history of Wisconsin, this is an incredible resource.
UWM Yearbooks - the yearbooks for UWM and its predecessors, dating from 1942 through 1968.
VADS - Online Resource for Visual Arts
Victoria and Albert Museum Fashion Exhibition - Spanning four centuries, the V&A’s Fashion collection is the largest and most comprehensive collection of dress in the world. Key items in the collection include rare 17th-century gowns, 18th-century ‘mantua’ dresses, 1930s eveningwear, 1960s daywear and post-war couture. Plus a growing number of pieces from 21st-century designers.
Kent State University has a guide like this one that has even more sources.